c. 1300, "ready and able to fight, able to defend," from Old French defensable, from Medieval Latin defensibilis, Late Latin defensibilem, from Latin defens-, past-participle stem of defendere (see defend). Meaning "capable of being defended" is from late 14c., sense of "contributing to defense" is from c. 1400; that of "that may be vindicated" is from early 15c. Related: Defensibility.
"to furnish with a battlement, render defensible," 1823, from crenel + -ate (2). Sometimes also crenellate; the double -l- seems to be from a presumed Latin *crenella as a diminutive of crena. Related: Crenelated, also crenellated; crenelation, crenellation (1849).
The Middle English verb for this was carnel (early 14c.), from the noun in Middle English and from Old French crenelé, from crenel.
1640s, "vindicatory, containing a defense," from French apologétique, from Latin apologeticus, from Greek apologetikos "defensible," from apologeisthai "speak in one's defense," from apologos "an account, story," from apo "away from, off" (see apo-) + logos "speech," from PIE root *leg- (1) "to collect, gather," with derivatives meaning "to speak (to 'pick out words')." Meaning "regretfully acknowledging failure" is by 1836 (apologetic for himself).
late Old English pil "sharp stake or stick," also, poetically, "arrow, dart," from Latin pilum, the name of the heavy javelin of the Roman foot soldier (source of Old Norse pila, Old High German pfil, German Pfeil "arrow"), a word of uncertain origin. De Vaan finds the identification of it with the pilum that means "pestle, pounder" (from *pis-tlo-, from the root of pinsere "to crush, pound;" see pestle) to be defensible.
In engineering and architecture, "a heavy timber beam, pointed or not, driven into the soil for support of a structure or as part of a wall." It also has meant "pointed head of a staff, pike, arrow, etc." (1590s) and the word is more or less confused with some of the sense under pile (n.1).